Abstract The photodegradation of polystyrene has been studied by electron spin resonance, infra-red absorption, viscosity and gel fraction measurements. Irradiations with light from a medium pressure mercury lamp to polystyrene film at 45°C in vacuum, gave rise to the polystyryl radical. When the irradiation was carried out in air under the same conditions as those in vacuum, the peroxy radical was formed. Main-chain scission of polystyrene occurs predominantly in air. Oxygenated products were detected by infra-red spectroscopic measurements and the degradation reaction was found to proceed via peroxide intermediates and result in main-chain scission leading to the formation of carbonyl compounds. When the polystyrene films were irradiated in vacuum, the main reaction was cross-linking through combination of polystyryl radicals. The photodegradation of polystyrene films containing benzophenone has also been investigated. This additive acts as a sensitiser for the photodegradation. A possible mechanism for the radical formation and degradation of polystyrene is proposed.